The production of hydrogen by the steam reforming of hydrocarbons is well known. In the basic process, a hydrocarbon, or a mixture of hydrocarbons, is initially treated to remove trace contaminants such as sulfur and olefins, which would adversely affect the reformer catalyst.
The pretreated hydrocarbon is typically compressed, e.g. to about 1.5 MPa to 3 MPa, and combined with high pressure steam, which is at about 5 MPa, before entering the reformer furnace. The reformer itself conventionally contains tubes packed with catalyst through which the steam/hydrocarbon mixture passes. An elevated temperature, e.g. about 860° C. is maintained to drive the reaction which is endothermic.
The effluent from the reformer furnace is principally H2, CO, and CO2 in proportion close to equilibrium amounts at the furnace temperature and pressure with a minor amount of methane. The reformate is conventionally introduced into a one- or two-stage shift reactor to form additional H2 and CO2. The shift reactor converts the CO to CO2 with the liberation of additional hydrogen by reaction at high temperature in the presence of steam. The combination of hydrocarbon/steam reformer and shift converter is well-known to those of ordinary skill in the art.
Various processes have been proposed to separate the effluent from the shift converter to recover hydrogen and CO2 therefrom.
In one such method, the shift converter effluent, which comprises H2, CO2, and H2O with minor quantities of CH4 and CO, is introduced into a chemical solvent-based adsorption unit selective for CO2. Such a unit operates on the well-known amine wash or Benfield processes wherein CO2 is removed from the effluent by dissolution in an absorbent solution, i.e. an amine solution or potassium carbonate solution, respectively. Conventionally, such units removed about 95 percent of the CO2 in the shift converter effluent.
In another method, cyclic pressure swing adsorption (PSA) systems or cyclic vacuum pressure swing (VPSA) systems are being employed to remove CO2 from shifted reformate streams. These systems are designed to fractionate gaseous mixtures by selective adsorption wherein the gaseous mixture is passed through a plurality of adsorption columns containing adsorbent beds which selectively retain CO2.
The present invention relates to CO2 separation by vacuum pressure swing adsorption techniques.
Related disclosures include U.S. Pat. Nos. 4,171,206, 4,299,596, 4,770,676, 4,790,858, 4,840,647, 4,857,083, 4,869,894, 4,913,709, 4,915,711, 4,963,339, 5,000,925, 5,026,406, 5,051,115, 5,133,785, 5,248,322, 5,354,346, 6,245,127, 7,550,030, 7,618,478, 7,740,688, and U.S. Pat. No. RE31014, each incorporated herein by reference.
CO2 produced in accordance with the present invention may be used for any desired purpose. For example, CO2 produced can be used for liquefaction to produce food-grade quality product(s), supercritical CO2 for enhanced oil recovery or simply CO2 for sequestration to avoid additional green house gases in the atmosphere in order to satisfy regulatory requirements.
Industry desires to separate CO2 from high pressure CO2-containing streams, for example reformate streams having a pressure ranging from 1 MPa to 7 MPa.
Industry desires increased recovery of CO2. Industry desires a CO2 recovery of greater than 90 mole %.
Industry desires high CO2 purity. Industry desires a CO2 purity in the CO2 product stream of at least 95 mole % on a dry basis.
Industry desires high CO2 purity from an adsorption system where no further purification other than condensation of water is required.
Industry desires to reduce compression costs for adsorption processes that separate CO2 from CO2-containing mixtures using pressure swing adsorption techniques.